Anti-reflection coatings (ARC) are widely used on optical surfaces (e.g., in lenses, prisms etc.) to suppress undesirable reflections. In general, such anti-reflection coatings are fabricated by a "dry" process, meaning non-liquid coating processes such as vacuum evaporation, sputtering or CVD (Chemical Vapor Deposition). See e.g. Joy George, Preparation of Thin Films (Marcel Dekker, Inc., New York, 1992) and Francois R. Flory, Thin Films for Optical Systems (Marcel Dekker, Inc., New York, 1995). In the case of a multilayer ARC, in order to obtain high performance (wide AR wavelength bandwidth, very low reflectance and wide angular AR bandwidth), it is well known that several kinds of coating materials which have respectively different refractive indices are needed. It is also known that the larger the difference of the refractive indices of the various coating materials, the better the optical performance, and the lower the refractive index of the lowest refractive index coating material used, the better the optical performance. Furthermore, it is possible to reduce the number of coating layers by use of both a large difference between the refractive indices of the various coating materials used and one of the various coating materials used having a very low refractive index.
However, for shorter wavelengths of incident light (e.g. near 200 nm), many kinds of coating materials cannot be used for an ARC which requires high transmittance, because of optical loss due to the light absorption by the coating material. Therefore, the number of coating materials which can be used at such short wavelengths (such as in the 200 nm ultra violet region) is limited, and so it is difficult to obtain a large enough difference between the refractive indices of the coating materials used, and to obtain a very low refractive index of one of the various coating materials used. Thus, it is difficult to design and fabricate high performance anti-refraction coatings with adequate performance for such wavelength regions.
For instance, for a typical ARC material formed by a dry process for the visible light region, a variety of ARC materials can be used. In general, in the visible light region, the highest refractive index material available is TiO.sub.2 (n=2.4 to 2.7 at 500 nm), and MgF.sub.2 (n=1.38 at 500 nm) is the lowest refractive index material available (where n is refractive index). For shorter wavelengths of incident light such as near 200 nm (ultra violet), however few ARC materials are available. In general, LaF.sub.3, NdF.sub.3 and GdF.sub.3 (all having n of about 1.7 at about 200 nm) are the highest refractive index coating materials available and Na.sub.3 A1F.sub.6 (n=1.36 at 200 nm) is the lowest refractive index coating material available. Therefore the difference between refractive indices of coating materials which can be used for the 200 nm wavelength region is much smaller than the difference of those for the visible light region. Those skilled in the art will readily appreciate that one consequence of the limited coating materials for short wavelength incident light is that ARC design and fabrication is much more difficult for the short wavelength region than for the visible light region.
In the case of an ARC for the visible light region, it is possible to obtain a low reflectance ARC having wide bandwidth that suppresses reflections over a large range of the visible light region (e.g. the reflectance is less than 0.5% from 400 to 800 nm wavelength.). However, many ARC layers (i.e. more than eight or nine) may be needed, extending manufacturing time increasing cost.
In the case of wet process anti-reflection coatings, such coatings are typically fabricated by the hydrolysis and the polymerization of a metal alkoxide solution, i.e., using a liquid; this wet process is called the sol-gel process. It is well known that optical coatings of SiO.sub.2, ZrO.sub.2, HfO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3, etc. can be fabricated not only by the dry process but also by the sol-gel process. See e.g. Ian M. Thomas, Applied optics Vol. 26, No. 21 (1987) pp. 4688-4691 and Ian M. Thomas, SPIE vol. 2288 Sol-Gel Optics III (1994) pp. 50-55. In the case of a SiO.sub.2 coating formed by the sol-gel process, the colloidal SiO.sub.2 suspensions that are suitable for the preparation of the SiO.sub.2 coating are usually prepared by the hydrolysis of silicon alkoxides in a parent alcohol as a solvent. The hydrolysis of tetraethyl silicate in ethanol, for instance, can be summarized by the following: EQU Si(OC.sub.2 H.sub.5)4+2H.sub.2 O.fwdarw.SiO.sub.2 +4C.sub.2 H.sub.5 OH
This reaction is complex and many variables such as catalyst, solvent, water ratio, and temperature all have an effect on the nature of the product. Furthermore, three types of liquid coating methods: spin, dip and meniscus are normally used to make wet process coatings. Spin and dip coating are common and widely used. Dip coating is preferred for large samples of irregular shape or having a curved surface. Spinning is excellent for small, round, flat or gently curved samples. Meniscus is particularly good for large, flat substrates. See e.g. Brinker and Scherer, Sol-Gel Science (Academic Press, Inc., San Diego, 1990); and Floch, Priotton and et al., Thin Solid Films, Vol. 175 (1989) pp. 173-178.
By using such a wet process, one generally obtains coatings which can have either high packing density and low packing density. In order to obtain wet process coatings with a high packing density equal to that of coatings formed by a dry process, it is necessary to heat the coating to high temperature (e.g. more than 450 degree C.), in the fabricating process. Not only does this result in a long manufacturing time and high cost, but also may result in damage or degradation to the substrate. However, it is easy to obtain low packing density coatings using the wet process, because an additional process such as heating is not required (since the process can be carried out at room temperature or less than 150 degrees C.).
Since the structure of coatings can be defined as micro-pores spacing apart deposited solid material, the relation between packing density and refractive index of optical coating is: EQU n.sub.f =n.sub.o .times.P+n.sub.p .times.(1-P)
Where n.sub.p is the refractive index of the material (e.g. air, water) which fills up the micro-pores, n.sub.f and n.sub.o are respectively the actual refractive index (dependent on packing density) and the refractive index of the deposited solid material, and P is the packing density of the coating. Furthermore packing density P is defined as:
P=(Volume of the solid part of the coating)/(Total volume of the coating (solid+pores)).
Thus, high and low packing density mean high and low refractive index respectively. In the case of a SiO.sub.2 coating fabricated by a wet process, the packing density can vary from 1 to approximately 0.5, and thereby the refractive index can vary from 1.45 to 1.22 in the visible light region. Thus, one obtains a nearly 0% reflectance single layer ARC on optical glass by using a wet process low packing density SiO.sub.2. This SiO.sub.2 ARC is also well known to have a high laser light damage threshold value. Therefore, this ARC is used for high power lasers, e.g. for nuclear fusion. See Ian M. Thomas, Applied Optics Vol. 31, No. 18 (1992) pp. 6145-6149.